Targeted drug delivery systems are a unique form of drug delivery where the pharmacologically active therapeutic agent is delivered only to its site of action and not to non-targeted sites. The goal of site specific drug delivery is to increase the selectivity and drug therapeutic index, and also to reduce the systemic toxicity of the drug. Therefore, targeted therapies provide a more promising alternative to circumvent the toxicities of conventional drugs, e.g., chemotherapeutic agents.
Various agents have been examined as targeting agents, including vitamins, carbohydrates, aptamers, peptides (e.g., Arg-Gly-Asp, allatostatin, transactivating transcriptional activator) and proteins (e.g., lectins, and transferrin). In addition, active agents, such as ligands for receptors and antibodies to surface proteins have been used extensively to target specific cells, but the majority of research to date has focused on antibodies.
Aptamers are DNA or RNA oligonucleotides that fold by intramolecular interaction into unique three dimensional conformations capable of binding to target antigens with high affinity and specificity. Aptamers exhibit many desirable properties for targeted drug delivery, such as ease of selection and synthesis, high binding affinity and specificity, low immunogenicity, and versatile synthetic accessibility. To date, a variety of anti-cancer agents (e.g. chemotherapy drugs, toxins, and siRNAs) have been successfully delivered to cancer cells in vitro using this technology. Aptamers possess several advantages over antibody ligands typically used in drug delivery. First, production of aptamers is easier to scale up with low batch-to-batch variability; second, aptamers are stable and can be denatured and renatured multiple times without loss of activity; third, the smaller size of aptamers relative to antibodies (˜150 kDa) can lead to better tissue penetration in solid tumors; fourth, lack of immunogenicity is a major advantage of aptamers over antibodies; fifth, conjugation chemistry for the attachment of various imaging labels or functional groups to aptamers is orthogonal to nucleic acid chemistry, and functionality can be introduced during aptamer synthesis. On the other hand, the disadvantages of aptamers include faster excretion than antibodies due to smaller size, weaker binding to targets, toxicity and other systemic properties, and susceptibility to serum degradation.
Nanoparticles have also recently emerged as a strategy for delivering therapeutic molecules effectively to targeted sites. Nanoparticles conjugated to a targeting ligand for effective drug delivery increases the chance of binding to surface receptors. However, safer and more effective drug conjugates are still desired. Further, the ability to assemble a molecule consisting of various moieties performing various functions, would provide a level of freedom for modifying the most fundamental properties of therapeutic drugs. Provided herein are solutions for these and other needs in the art.